It might seem an innocuous move to outsiders but in China it was a game-changer. Authorities in Shanghai began encouraging newly married couples in the city to have two children.

The rationale: Under China’s vaunted one-child policy – a cornerstone of economic and social planning for decades – the population has been aging too rapidly. Indeed, Shanghai, which has always had a relatively youthful populace, now has the same proportion of retirees as an average city in the United States or Europe.

The move last summer by the city of Shanghai marked the first time since 1979 that officials have exhorted couples to have more offspring. More important, it symbolizes a sharpening debate in the world’s most populous country over one of Beijing’s most fundamental totems.

A growing chorus of critics is warning that unless the government changes course, the nation’s one-child policy will drive the Asian powerhouse into a demographic dead end. They see China growing old before it grows rich.

Officials are beginning to take note. Spooked by the prospect of only 1.7 active workers for every pensioner by 2050, they are quietly chipping away at Beijing’s signature population edict.

They have another reason to worry, too – forecasts that within 30 years, 15 percent of marriage-aged men will be unable to find brides. The combination of the one-child policy and the Chinese preference for male offspring has proved deadly for female fetuses: 120 boys are born for every 100 girls – the highest ratio in the world.

“This will cause a grave humanitarian disaster,” predicts Mu Guangzong, a professor at Peking University’s Institute of Population Research.

The one-child policy has always been controversial abroad and unpopular at home. More than 70 percent of Chinese women would like to have two or more children, a study released earlier this year by the National Family Planning Commission found.

It is nonetheless a policy to which the government has attached fundamental importance since it was written into the Constitution in 1978. Officials say it has prevented 400 million births, and raised living standards for the children that were born.

Maybe. But among the unintended consequences, or ones that were simply ignored, is a population aging twice as fast as America’s. There will be 400 million people over 65 by 2040 – a quarter of the population – estimates Chen Wei, an expert at Renmin University’s Population and Development Research Institute.

Not only will their pensions and healthcare become a “very severe burden on the government budget,” Professor Chen warns, but there will also be fewer working-age citizens to support them. And those still working will be older, and thus less productive, than today’s labor force. While China’s population growth has until now boosted economic growth, it will act as a drag when the labor force starts shrinking after 2020, economists predict.

The government is working on several fronts to head off disaster. It is setting up a rudimentary pension system for rural dwellers who still do not have any social security. It is paying lifetime grants to parents with only girl children. It is encouraging Chinese farmers to value their womenfolk more highly.

It is also listening to experts such as Hu Angang, a government adviser, who argues that “now is the right time for us to change the family planning policy. The longer we wait, the higher the cost will be.”

Still, any changes in policy will likely be incremental. Under the edict in Shanghai, for instance, the only couples who can have a second child are those who are only children themselves. The next step, says Chen, will be to allow couples to have two babies if either the mother or the father are only children.

“That will happen soon,” he says, perhaps when the next Five Year Plan is launched in 2011. Within a decade or so, he adds, China will have a two-child policy.

Yet for now, the one-child policy is sacred enough that no one predicts it will be ditched wholesale. “The regime has staked its credibility on the correctness of the one-child policy,” says Susan Greenhalgh, an expert on Chinese population issues at the University of California, Irvine. “It would be very, very difficult politically to announce that it has been abandoned.”

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